Question

The reaction of zinc with excess of aqueous alkali, evolves hydrogen gas and gives:

• A. Zn(OH)₂
• B. ZnO
• C. [Zn(OH)₄]^2–
• D. [ZnO₂]^2–

Answer 1

The Correct Option is D

To solve this question, we need to understand the chemical reaction of zinc with aqueous alkali. When zinc is treated with excess aqueous alkali (such as NaOH or KOH), it reacts in the following manner:

The general reaction can be represented as:

\text{Zn} + 2\text{OH}^- + 2\text{H}_2\text{O} \rightarrow \left[\text{Zn(OH)}_4\right]^{2-} + \text{H}_2 \uparrow

However, in very strong solutions or with excess alkali, the zincate ion is more often represented as:

\text{Zn} + 4\text{OH}^- \rightarrow \left[\text{ZnO}_2\right]^{2-} + 2\text{H}_2\text{O} + \text{H}_2 \uparrow

In the above reaction, \left[\text{ZnO}_2\right]^{2-} is the zincate ion produced when zinc reacts with an excess of a strong alkali solution, evolving hydrogen gas. This is why \left[\text{ZnO}_2\right]^{2-} is the correct answer.

Let's now evaluate the given options to understand why \left[\text{ZnO}_2\right]^{2-} is the correct answer:

1. Option \text{Zn(OH)}_2 represents zinc hydroxide, which is an intermediate compound in basic conditions and does not evolve hydrogen gas directly because it needs an alkali for complete dissolution.
2. Option \text{ZnO} is just zinc oxide and does not involve hydrogen gas formation directly in the chemical process being described.
3. Option \left[\text{Zn(OH)}_4\right]^{2-} is also a zincate complex but less commonly referenced than \left[\text{ZnO}_2\right]^{2-} under conditions of excess alkali when the zincate form predominates.
4. Option \left[\text{ZnO}_2\right]^{2-} is the most representative form when zinc is reacted with excess alkali.

Therefore, the correct answer is option \left[\text{ZnO}_2\right]^{2-}, because it correctly identifies the product when zinc reacts with an excess aqueous alkali producing hydrogen gas.